In recent years, internal combustion engine makers, and in particular diesel engine manufacturers, have been faced with ever increasing regulatory requirements, namely exhaust emissions regulations. Exhaust emissions takes on a number of forms including visible smoke, particulate matter and oxides of nitrogen (NOx). As is generally know in the art, particulate matter is comprised of mainly unburned hydrocarbons and soot whereas NOx is an uncertain mixture of oxides of nitrogen (mainly NO and some NO.sub.2). To address these emissions issues, different technologies have been developed or used, including fuel injection and combustion control strategies and systems, after-treatment systems, exhaust gas recirculation (EGR) systems, and, in some cases intake air separation systems.
Many emission reduction systems have a negative effect on fuel efficiency, an issue that is of great importance to most users of diesel engines. One well-known method of improving engine fuel efficiency or power density is by increasing the amount of oxygen in the cylinder. Typically this has been accomplished by pressurizing the air taken into the combustion chamber. The main goal of this pressurization is to increase the oxygen available for combustion. Others have increased the concentration of oxygen in the combustion air using air separation techniques. See, for example, U.S. Pat. No. 5,649,517 (Poola et al.) issued on Jul. 22, 1997 and U.S. Pat. No. 5,636,619 (Poola et al.) issued on Jun. 10, 1997 which disclose the use of a semi-permeable gas membrane on a portion of the intake air to reduce the nitrogen content from the intake air flow to create an oxygen enriched air supply for combustion purposes. The '517 patent also discloses potential uses for the nitrogen enriched air stream exiting the air separation device. Another related art disclosure of interest is U.S. Pat. No. 5,553,591 (Yi) issued to on Sep. 10, 1996 which shows a vortex air separation system for creating oxygen enriched intake air to increase the power generated during combustion. Still other related art systems employing oxygen enrichment are disclosed in U.S. Pat. No. 5,400,746 (Susa et al.) issued on Mar. 28, 1995 and U.S. Pat. No. 5,678,526 issued on Oct. 21, 1997. See also U.S. Pat. Nos. 5,051,113 and 5,051,114 (Nesmer et al.)
It is well known that the introduction of oxygen enriched intake air during the intake stroke of facilitates burning a larger part of the available fuel injected which in turn increases the power output for each combustion cycle or charge, and generally reduces brake specific fuel consumption (BSFC). Lower BSFC correlates strongly with reduction in unburned fuel and overall improvement in fuel economy.
Other related art disclosures include U.S. Pat. No. 5,526,641 (Sekar et al.) and U.S. Pat. No. 5,640,845 (Ng et al.) which disclose similar air separation techniques for creating oxygen enriched air as well as nitrogen enriched air specifically for after-treatment purposes. Utilization of an air separation system has also been tried for the purpose of reducing emissions such as particulates and NOx. See K. Stork and R. Poola publication "Membrane-Based Air Composition Control for Light Duty Diesel Vehicles" (October 19998). Most particulates generated during the combustion cycle form relatively early in the combustion cycle, but such early forming particulates usually burn as temperature and pressure increase during the combustion cycle. The particulates that typically enter the exhaust stream tend to form in the latter part of the combustion cycle as the pressure and temperature decreases. In addition to decreasing BSFC, increasing air intake oxygen content serves to reduce the quantity of unburned hydrocarbons by increasing the likelihood of complete combustion.
After-treatment of exhaust gas is useful in reducing the amount of unburned hydrocarbons. After-treatment methods take steps to continue the oxidation of the unburned hydrocarbons. One manner is by introducing a secondary air supply into the exhaust stream. This secondary air stream provides more oxygen to the already high temperature exhaust ensuring further oxidation. While using secondary air is effective in eliminating particulates, the further oxidation creates still higher temperatures in the exhaust system. Designing the exhaust system for these higher temperatures requires components able to withstand the hotter environment. These components often times are heavier, expensive or require more frequent servicing.
While particulate production generally decreases along with fuel consumption, NOx production generally increases. NOx forms where nitrogen mixes in a high temperature setting with excess oxygen not used in the combustion process. Thus, while excess oxygen and high combustion temperatures are beneficial in reducing fuel consumption, such combination is detrimental in terms of increased NOx formation. This conflict generally leads engine manufacturers to delicately balance NOx production with brake specific fuel consumption (BSFC) and particulate matter in order to meet emission regulations. The present invention resolves, at least in part, the continuing conflict between reducing particulates, reducing NOx, and decreasing BSFC.
Exhaust Gas Recirculation (EGR) is one technique currently in use to reduce NOx formation within the combustion cylinder. EGR reduces the amount of available oxygen for formation of NOx. By reducing the amount of oxygen, the combustion process is also slowed thereby reducing the peak temperatures in the combustion chamber. EGR systems typically use exhaust gas, however the '517 patent (Poola et al.) shows using an enriched nitrogen source extracted from a portion of the intake air instead of recirculated exhaust gas to displace oxygen in the combustion chamber. See also K. Stork and R. Poola publication "Membrane-Based Air Composition Control for Light Duty Diesel Vehicles" (October 1998). The enriched nitrogen air is both cleaner and cooler than exhaust gas, and thus provides distinct advantages over exhaust gas recirculation.
From the above discussion it appears well known that oxygen enriched air and nitrogen enriched air have a number of beneficial uses within an internal combustion engine and a diesel engine in particular. What is needed therefor are various improvements to the existing air separation systems so that such systems are useful in a heavy-duty diesel engine or similar such application. More importantly, what is needed are improvements to such existing air separation systems that provide reliable and durable designs of an intake air separation system and that effectively balances the fuel consumption requirements and emissions. Such a system should be simple and relatively inexpensive to manufacture, install, operate and maintain. The present invention is directed at overcoming one or more of the problems set forth above.